feat(session-historian): cross-platform session history agent and /ce-sessions skill (#534)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

plugins/compound-engineering/skills/ce-sessions/SKILL.md

@@ -0,0 +1,33 @@
+---
+name: ce-sessions
+description: "Search and ask questions about your coding agent session history. Use when asking what you worked on, what was tried before, how a problem was investigated across sessions, what happened recently, or any question about past agent sessions. Also use when the user references prior sessions, previous attempts, or past investigations — even without saying 'sessions' explicitly."
+---
+
+# /ce-sessions
+
+Search your session history.
+
+## Usage
+
+```
+/ce-sessions [question or topic]
+/ce-sessions
+```
+
+## Pre-resolved context
+
+**Repo name (pre-resolved):** !`common=$(git rev-parse --git-common-dir 2>/dev/null); if [ "$common" = ".git" ]; then basename "$(git rev-parse --show-toplevel 2>/dev/null)"; else basename "$(dirname "$common")"; fi`
+
+**Git branch (pre-resolved):** !`git rev-parse --abbrev-ref HEAD 2>/dev/null`
+
+If the lines above resolved to plain values (a folder name like `my-repo` and a branch name like `feat/my-branch`), they are ready to pass to the agent. If they still contain backtick command strings or are empty, they did not resolve — omit them from the dispatch and let the agent derive them at runtime.
+
+## Execution
+
+If no argument is provided, ask what the user wants to know about their session history. Use the platform's blocking question tool (`AskUserQuestion` in Claude Code, `request_user_input` in Codex, `ask_user` in Gemini). If no question tool is available, ask in plain text and wait for a reply.
+
+Dispatch `compound-engineering:research:session-historian` with the user's question as the task prompt. Omit the `mode` parameter so the user's configured permission settings apply. Include in the dispatch prompt:
+
+- The user's question
+- The current working directory
+- The repo name and git branch from pre-resolved context (only if they resolved to plain values — do not pass literal command strings)